Custom widget
Have you ever looked at an application and wondered, how a particular gui item was created? Probably every wannabe programmer has. Then you were looking at a list of widgets provided by your favourite gui library. But you couldn't find it. Toolkits usually provide only the most common widgets like buttons, text widgets, sliders etc. No toolkit can provide all possible widgets.
There are actually two kinds of toolkits. Spartan toolkits and heavy weight toolkits. The FLTK toolkit is a kind of a spartan toolkit. It provides only the very basic widgets and assumes, that the programemer will create the more complicated ones himself. wxWidgets is a heavy weight one. It has lots of widgets. Yet it does not provide the more specialized widgets. For example a speed meter widget, a widget that measures the capacity of a CD to be burned (found e.g. in nero). Toolkits also don't have usually charts.
Programmers must create such widgets by themselves. They do it by using the drawing tools provided by the toolkit. There are two possibilities. A programmer can modify or enhance an existing widget. Or he can create a custom widget from scratch.
Burning widget
This is an example of a widget, that we create from scratch. This widget can be found in various media burning applications, like Nero Burning ROM.
custom.rb#!/usr/bin/ruby # ZetCode Ruby GTK tutorial # # This example creates a burning # custom widget # # author: jan bodnar # website: zetcode.com # last edited: June 2009 require 'gtk2' class Burning < Gtk::DrawingArea def initialize(parent) @parent = parent super() @num = [ "75", "150", "225", "300", "375", "450", "525", "600", "675" ] set_size_request 1, 30 signal_connect "expose-event" do expose end end def expose cr = window.create_cairo_context draw_widget cr end def draw_widget cr cr.set_line_width 0.8 cr.select_font_face("Courier", Cairo::FONT_SLANT_NORMAL, Cairo::FONT_WEIGHT_NORMAL) cr.set_font_size 11 width = allocation.width @cur_width = @parent.get_cur_value step = (width / 10.0).round till = (width / 750.0) * @cur_width full = (width / 750.0) * 700 if @cur_width >= 700 cr.set_source_rgb(1.0, 1.0, 0.72) cr.rectangle(0, 0, full, 30) cr.clip cr.paint cr.reset_clip cr.set_source_rgb(1.0, 0.68, 0.68) cr.rectangle(full, 0, till-full, 30) cr.clip cr.paint cr.reset_clip else cr.set_source_rgb 1.0, 1.0, 0.72 cr.rectangle 0, 0, till, 30 cr.clip cr.paint cr.reset_clip end cr.set_source_rgb(0.35, 0.31, 0.24) for i in (1..@num.length) cr.move_to i*step, 0 cr.line_to i*step, 5 cr.stroke te = cr.text_extents @num[i-1] cr.move_to i*step-te.width/2, 15 cr.text_path @num[i-1] cr.stroke end end end class RubyApp < Gtk::Window def initialize super set_title "Burning" signal_connect "destroy" do Gtk.main_quit end set_size_request 350, 200 set_window_position Gtk::Window::POS_CENTER @cur_value = 0 vbox = Gtk::VBox.new false, 2 scale = Gtk::HScale.new scale.set_range 0, 750 scale.set_digits 0 scale.set_size_request 160, 35 scale.set_value @cur_value scale.signal_connect "value-changed" do |w| on_changed(w) end fix = Gtk::Fixed.new fix.put scale, 50, 50 vbox.pack_start fix @burning = Burning.new(self) vbox.pack_start @burning, false, false, 0 add vbox show_all end def on_changed widget @cur_value = widget.value @burning.queue_draw end def get_cur_value return @cur_value end end Gtk.init window = RubyApp.new Gtk.main
We put a DrawingArea on the bottom of the window and draw the entire widget manually. All the important code resides in the draw_widget which is called from the expose method of the Burning class. This widget shows graphically the total capacity of a medium and the free space available to us. The widget is controlled by a scale widget. The minimum value of our custom widget is 0, the maximum is 750. If we reach value 700, we began drawing in red colour. This normally indicates overburning.
@num = [ "75", "150", "225", "300",
"375", "450", "525", "600", "675" ]
These numbers are shown on the burning widget. They show the capacity of the medium.
@cur_width = @parent.get_cur_value
From the parent widget, we get the current value of the scale widget.
till = (width / 750.0) * @cur_width full = (width / 750.0) * 700
We use the width variable to do the transformations. Between the values of the scale and the custom widget's measures. Note that we use floating point values. We get greater precision in drawing. The till parameter determines the total size to be drawn. This value comes from the slider widget. It is a proportion of the whole area. The full parameter determines the point, where we begin to draw in red color.
cr.set_source_rgb(1.0, 1.0, 0.72) cr.rectangle(0, 0, full, 30) cr.clip cr.paint cr.reset_clip
We draw a yellow rectangle up to point, where the medium is full.
te = cr.text_extents @num[i-1] cr.move_to i*step-te.width/2, 15 cr.text_path @num[i-1] cr.stroke
This code here draws the numbers on the burning widget. We calculate the text extents to position the text correctly.
def on_changed widget
@cur_value = widget.value
@burning.queue_draw
end
We get the value from the scale widget, store it in the @cur_value variable for later use. We redraw the burning widget.
In this chapter, we created a custom widget in GTK and Ruby programming language.